1. Field of the Invention
The present invention relates to a decoder and a reproducing device for decoding by propagating reliability information between an internal decoder and an external decoder, and more particularly to a decoder and information reproducing device for detecting a defective section from a signal read from medium and manipulating reliability information of an internal decoder.
2. Description of the Related Art
Recently digital signal processing systems, which perform iterative decoding using turbo codes for propagating reliability and low density parity check (LDPC) codes, are receiving attention.
For example, errors which occur in a storage medium, such as a magnetic disk, are roughly divided into random errors and burst errors. The random errors are errors which are distributed over a wide range without long continuance. Whereas the burst errors are continuous errors which are generated by a defect in the media and thermal asperity (TA), which is unique to hard disk drives.
The major characteristic of a iterative decoding system is that erred information in codes which have largely relation acquired from large blocks and randomness, can be corrected by a plurality of correct information at a distance position based on the propagation of reliability information. A iterative decoding system has high capacity to correct the randomly distributed information by the reliability propagation.
FIG. 16 is a block diagram depicting a reproducing device using conventional iterative decoding. As FIG. 16 shows, the iterative decoding system has a plurality of decoders (e.g. soft-input soft-output (SISO) decoder 642 for PR channel, and belief propagation (BP) decoder 644 for low density parity check code), as a iterative decoder 640. And errors are corrected by mutually propagating the reliability information (also called “likelihood information”) “0” and “1”. Some types of iterative decoders perform iterative decoding only within the belief propagation (BP) decoder 644.
In other words, a reproducing waveform reproduced from the recording medium via a head receives a known inter-signal interference from a PR waveform equalizer 602. In other words, in the PR waveform equalizer 602, the reproducing waveform passes through an analog filter 610 and is input to an analog/digital converter (A/D converter) 612. The A/D converter 612 performs digital sampling according to a sampling clock. A digital-sampled sampling waveform passes through a digital filter 614 and is equalized to a desired partial response (PR), and a PR equalized series is obtained.
A first decoder (SISO decoder) 642 for the PR channel outputs the most likely reliability information corresponding to recorded data “0” and “1”. Also based on the check information (e.g. parity bit) added to the recorded data, a second decoder (BP decoder) 644 performs error check, and updates the reliability information.
Then the updated reliability information is fed back to the first decoder 642. This iterative operation is performed under predetermined conditions, and finally the reliability information is judged as binary data “0” and “1”, and iterative decoding completes. The decoded data after the iterative decoding is error-corrected by an error correction unit 630, such as an ECC correction circuit, and a decoded data string is acquired.
However if a burst error occurs in the recoding device due to a drop in signal amplitude, which is generated by a defect in the medium, incorrect reliability information is randomly dispersed and propagated, which spreads error information and makes decoding impossible.
To handle this burst error, a thermal asperity detector 620 is installed, so that the reproducing waveform is judged by a fixed threshold value, thermal asperity is detected, and a erasure flag is output. A method for suppressing the propagation of incorrect reliability information by inputting this erasure flag to a first decoder 642 and setting the reliability information in this position to “0” (undefined) has been proposed (e.g. Japanese Patent Application Laid-Open No. 2004-127408).
Another method under proposal is that when RLL (Run Length limited) code modulation is used, a constraint violation of the RLL code is recognized by the output of a first decoder 642, so as to specify the location of the burst error, and the reliability information, which is the output of the first decoder 642, is regarded as undefined (e.g. Japanese Patent Application Laid-Open No. 2005-166089).
Although this is not a method for handling defects of a medium, a method of correcting the branch metrics in a maximum-likelihood decoder according to an instantaneous drop in the amplitude of a reproducing signal in a communication path has also been proposed (Japanese Patent Application Laid-Open No. 2002-164946).
However, in the case of the method of correcting the reliability information of the iterative decoder in a conventional iterative decoding system, which targets a burst error, the reliability information is set to “0” (undefined) only when a clear amplitude abnormality (e.g. drop, level saturation) is generated.
A defect of a medium, however, is not always one that spreads over a wide area in this way, but may be a local defect (called a “micro defect”). This micro defect could possibly cause serious error propagation, and with prior art, it is difficult to correct reliability information when such a micro defect occurs. Also the reliability information is set to “0” when a defect is detected, so it is difficult to effectively correct reliability information for a micro defect.
Also in the case of a prior art targeting reproducing signals on a communication path, if inter-symbol interference occurs in signals to be reproduced and if multi-value levels must be detected, that is, if channel reproducing signals having inter-symbol interference, such as PR signals, are decoded, correct identification point signals may be corrected by mistake, which deteriorates performance.